1. Field of the Invention
The present invention relates to an ultrasonic measuring apparatus of the type having a pair of transducers disposed oppositely on a pipe through which a fluid to be measured flows and which are so controlled alternately as to convert an input electrical signal to an acoustic signal and to convert a received acoustic signal to a received electrical signal, oscillating means having a variable oscillating frequency, a counter for counting the output signal from the oscillating means to produce an output signal when the count value of the counter reaches a predetermined value, a delay means for delaying the output signal from the counter, a transmitter receiving the output signal from the oscillating means for producing the input electrical signal, a time difference detecting means receiving the output signal from the delay means and the received electrical signal for detecting the time difference between the delayed counting time obtained from the delay means corresponding to the counting time that the counter counts the output signal from the oscillating means up to the predetermined value and the propagating time required for an ultrasonic wave to propagate through the fluid to be measured, controlling means for controlling the oscillating frequency of the oscillating means so as to make the time difference zero, and means for detecting the propagating time in accordance with the oscillating frequency of the oscillating means. A frequency difference between the oscillating frequency when the ultrasonic wave is emitted into the flow in a forward direction with respect to the flow direction and the oscillating frequency when it is emitted in a backward direction with respect to the flow direction is used to measure the flow speed or flow rate.
2. Prior Art
An ultrasonic flow rate measuring apparatus of this type is disclosed in an article "Fuji Ultrasonic Flowmeter" on pp 29 to 38 of "Fuji Giho" published by Fuji Electric Co., Ltd., Vol. 48, No. 2. The construction of this conventional ultrasonic flow rate measuring apparatus is shown in FIG. 1. In FIG. 1, reference numeral 10 designates a measuring pipe through which a fluid flows in the direction of the arrow. Mounting elements 15 and 16 mount transducers 13 and 14 on the outer surface of the pipe 10.
The transducers 13 and 14 convert acoustic signals into electric signals and vice versa. In one operation mode, the transducer 13 serves as a transmitter and the transducer 14 serves as a receiver. In another operation mode, the transducer 14 operates as an acoustic transmitter and the transducer 13 as an acoustic receiver. These modes are changed one to the other by a mode changer 9. The mode changer 9 produces mode switch signals A and B, which control the transducers 13 and 14 via a gate circuit 6 so as to alternately serve as the transmitter and the receiver.
In the figure, reference numeral 1 designates an oscillator unit having a pair of voltage controlled oscillators 11 and 12 and controllers 19 and 20. In the oscillator unit 1, the control voltages are varied by the controllers 19 and 20 in accordance with the output signal from a time difference detector 8 so that the oscillating frequencies are varied. The mode change signals A and B also control the controllers 19 and 20 in such a way that, in a given mode, one of the oscillators 11 and 12 accepts the output signal from the time difference detector 8. Reference numeral 2 designates a synchronizing (sync) pulse generator for producing a signal in synchronism with the output signal of the oscillator 11 or 12 selected by the mode changer 9. Reference numeral 3 designates a counter for counting the output signal from the oscillator unit 1. The counter 3 starts its counting operation in response to the output signal from the synchronizing pulse generator 2. When the count value of the counter 3 reaches a given value N, it produces a count operation end signal. Reference numeral 4 designates a delay circuit which initiates its operation in response to the output signal from the counter 3 to produce an output signal after a given time lapse. The delay time .tau..sub.d of the delay circuit 4 is substantially equal to the total time .tau. of the time taken for an ultrasonic wave propagating through the measuring pipe 10 and the time taken for an electrical signal to pass through a transmitter 5 and a receiver 7. The output signal V from the delay circuit 4 is led to the time difference detector 8. The transmitter 5 transmits an electrical signal to drive the transducers 13 and 14 on the basis of the output signal from the sync pulse generator 2. The output signal from the transmitter 5 is selectively applied through the gate circuit 6 to the transducer 13 or 14. Similarly, the receiving signal received by the transducer 13 or 14 is led through the gate 6 to the receiver 7.
The receiver 7, upon the detection of the receiving signal, produces a trigger signal Z to control the time difference detection circuit 8. As shown in FIG. 2 illustrating a circuit diagram of the time difference detector 8, a NAND gate 100 is provided in the prestage of the circuit. To the NAND circuit 100 a trigger signal Z from the receiver 7 and an output signal V from the delay circuit 4 are applied. When those signals Z and V coincide with each other causing the NAND gate 100 produce an output signal F of logical "1", the transistor Q1 which has been conductive is turned off so that charge current flows from a constant current circuit 90 into a capacitor through a diode D1 to charge the capacitor C. The constant current circuit 90, the transistor Q1, the diode D1 and the capacitor C cooperatively form a ramp circuit. The output signal of the ramp circuit, i.e., the charge voltage across the capacitor C, is led to a differential amplifier 80. A set voltage E50 for measuring the propagation time is preset in the differential amplifier 80 and the difference voltage between the output signal R from the ramp circuit and the set voltage E50 is derived as an output signal S from the time difference detector 8. This output signal S is led to the controllers 19 and 20 in the oscillator unit 1. The control voltages of the oscillators 11 and 12 are so controlled that the difference voltage becomes zero. As a result, the oscillating frequency is varied and the varied frequency is counted by the counter 17 to be converted into a propagation time t which is then applied to a display unit 18. A field effect transistor Q2 may be used to provide a path for discharging the capacitor C. The field effect transistor Q2 is controlled to perform on-off switching by a signal K. In this embodiment, the set voltage E50 may be selected to be approximately 5 V.
In operation, it is assumed that the mode change signal A is first produced from the mode changer 9 and accordingly the transducer 14 is forced to operate as the receiver and the transducer 13 is forced to operate as the transmitter, while at the same time the oscillator 11 in the oscillator unit 1 is connected to the sync pulse generator 2 and the counter 3. The gate circuit 6 is so controlled that the output signal from the transmitter 5 is led to the transducer 13 and that the output signal from the transducer 14 is led to the receiver 7. After the lapse of a given time, the delay circuit 4 produces the output signal V which interrupts the production of an output signal F from the NAND circuit 100. Accordingly, the capacitor C of the ramp circuit starts its charging. Then, the output signal is generated from the transducer 14. When the impingement of the ultrasonic wave on the transducer 14 is detected by the receiver 7, the receiver 7 stops generation of the output signal Z, so that the output signal F from the NAND circuit 100 is again produced, leading to the interruption of the charging into the capacitor C. The output signal from the ramp circuit at this time is denoted as R' hereinafter. The output signal R' is compared with the set voltage E50 and a resultant difference voltage .epsilon. therebetween is produced as an output signal S of the time difference detector 8. The oscillating frequency of the oscillator 11 is controlled by the difference voltage .epsilon.. Through the repetition of such an operation, the difference voltage finally becomes zero, that is to say, the output signal R' is controlled to be equal to the set voltage E50. In this manner, when the ultrasonic pulse is emitted into the flowing fluid in the forward direction with respect to the fluid flow, the forward propagation time Ta is transformed into a corresponding oscillating frequency of the oscillator 11. In this way, the forward direction propagation time measurement is completed.
Then, the mode changer 9 produces the mode change signal B. As a result, the transducer 14 is changed to operate as the transmitter while the transducer 13 is changed to operate as the receiver. At the same time the signal B controls the oscillator 12 in the oscillator unit 1 to be connected to the sync pulse generator 2 and the counter 3. The gate circuit 6 is so controlled that the output signal from the transmitter 5 is led to the transducer 14 and the output signal from the transducer 13 is led to the receiver 7. When the ultrasonic pulses are emitted into the flowing fluid in the backward direction with respect to the fluid flow, in the same manner as mentioned above, the backward propagation time Tb is transformed into a corresponding oscillating frequency of the oscillator 12. At this point, the measurement of the backward propagation time is completed.
The frequency difference between the frequencies of the oscillators 11 and 12 is derived as a frequency difference proportional to the flow speed from the reversible counter 17 and is applied to the display unit 18 where this difference is displayed as the flow rate or flow speed.
In FIG. 1, l and t related to the measuring pipe 10 are representative of the propagation distance of the ultrasonic wave in the measuring medium and the propagation time, respectively. In the apparatus shown in FIG. 1, the counting time required for the counter 3 to count N of the oscillating frequency f of the oscillator unit 1, is expressed by N/f. In this example, a feedback loop is formed so that this counting time coincides with the acoustic wave propagation time t in the measuring medium. Therefore, at the time point that the propagation becomes stable, N/f=t, i.e. f=N/t is obtained and therefore the frequency f thus derived is N times as long as the reciprocal of the propagation time, l/t. Accordingly, the propagation time t may be measured by counting the frequency f.
The ultrasonic wave is easily affected by the conditions of its propagation path. Therefore, it is necessary to carefully monitor its propagation, that is to say, whether the ultrasonic wave propagates normally or not. For such a monitoring, a conventional technique has employed the use of a signal monitor circuit (see Japanese Laid-Open patent application No. 101,668/1976) or an abnormality value removal circuit.
The above-mentioned signal monitor circuit judges whether the peak value of the receiving waveform falls within a given range or not. However, when the receiving waveform is distorted by, for example, the absorption of the ultrasonic pulse by bubbles in the flowing fluid, an erroneous signal may be generated indicating that the ultrasonic pulse has reached the receiving transducer; that is to say, a so-called mistriggering takes place. In the latter case of the abnormal value removal circuit, when the final output value (an average value over 5 seconds) is considerably different from the previous output value (an average value over 5 seconds prior to the final output), that value is judged to be abnormal. Accordingly, the final value is not derived as data while its previous value is held. The response of the flow rate measurement, therefore, is slow. Further, a sudden change of the output signal caused by a sudden change of flow speed or level might be misjudged as an abnormal situation.